A common goal in laser systems is to provide devices with good beam quality that operate in an efficient manner. One technique for generating radiation having good transverse beam quality is to use a laser resonator that is relatively small in its transverse dimensions so that only diffraction limited or near diffraction limited transverse modes are supported by the resonator. In conventional rod-shaped laser resonators, this may be accomplished using pinholes or apertures in the laser cavity, or by only pumping or exciting a very limited transverse area in the gain medium. However, one drawback of this rod laser approach, in which only a limited transverse gain region is used, is that the small volume effectively limits the amount of energy that can be stored or the power that can be extracted by the laser system. In those cases in which pinholes are not used, but the excitation of the medium is limited to a small transverse area, one drawback is the difficulty encountered in designing an excitation source. In general, such small-excited volume systems use an end-pumping geometry in which a laser provides an excitation source to deliver the pump power into a well-defined region through the end of the laser rod. This approach similarly suffers from energy storage and extracted power limitations. In laser resonators that extract high beam quality radiation from a transverse region that is large compared to that which would be required to support only a diffraction limited mode, it may be possible to use an unstable resonator. The unstable resonator structure, however, exhibits high diffractive losses, and is thus limited in general to high gain laser systems.
Guided wave optics is largely based on the physical phenomenon of total internal reflection. It has important applications in the fabrication of miniaturized optical and opto-electronic devices where light radiation is confined within a defined region. An optical waveguide may be considered a light conduit consisting of a slab, strip or cylinder of dielectric material surrounded by materials with a lower refractive index. Optical dielectric waveguides are generally constructed of a guiding layer, a substrate and a cover layer. The guiding layer has a higher refractive index than the substrate and the cover layer to confine optical radiation by total reflection upon the walls separating it from the other two. Optical waveguides are discussed in "Fundamentals of Photonics" by B. E. A. Saleh and M. C. Teich (John Wiley 1991). In particular, a planar dielectric waveguide may be considered a conceptually simple two-dimensional design formed with a central slab of a medium of propagation with a higher refractive index. This guiding slab may also be described as a core. The upper and lower media of the structure are generally formed of a material with a lower refractive index. The lower medium may also be characterized as a substrate. When the upper medium has the same refractive index as the lower one, then the waveguide may be referred to as symmetrical. In addition, the upper medium may be simply surrounding air or another medium with a refractive index that is different from the substrate. The planar waveguide may be referred to as asymmetrical in these instances. A planar dielectric guide, however, generally does not provide confinement along the plane of the substrate. Dielectric strip or channel waveguides, which are also called three-dimensional guides, can provide this additional type of radiation confinement. Among strip waveguides, the most common includes those that are formed with a raised strip, an embedded strip and a rib or ridge or a strip loaded guide.
A waveguide laser generally includes a guiding layer with at least one lasant ion. By optically confining both the pump radiation and the developed laser radiation to very thin structures, sufficient gain may still be generated from many ion-host combinations. The resulting gain, which may be offered by a particular waveguide configuration, enables a range of optical resonator designs that are conducive to producing near diffraction limited and diffraction limited output beams. For example, U.S. Pat. No. 4,679,892 describes passive and active waveguide components and devices produced by liquid phase epitaxy and photolithography with garnet crystals of different refractive index. This technique may be particularly applicable to magneto-optical devices. In addition, U.S. Pat. No. 5,502,737 describes the fabrication of waveguide laser structures by epitaxial growth of crystal layers from a liquid phase. Although this technique may be applied to the fabrication of planar waveguides, it imposes serious limitations on the choice of waveguide core and substrate material combinations due to lattice matching requirements for liquid phase epitaxy. To increase the refractive index of the guiding core, it may be necessary to co-dope the flux of it with ions such as Ga, Ge or Lu. This may undesirably alter the spectroscopic lasant properties of the core crystal, as by way of example manifested as line broadening. It may be necessary to match the crystal structure of substrate and guiding layers, thus making it particularly difficult to design waveguide lasers with a large numerical aperture that have the same desirable lasant characteristics as a bulk rod or slab laser. While waveguides of dissimilar materials such as single crystalline silicon on sapphire are known and have been used for the fabrication of modulators and switches, as described in U.S. Pat. No. 4,904,039, analogous structures of guiding layers of dielectric crystals or glasses for solid state lasers cannot be grown in general, presumably because stresses between these dissimilar materials may not provide stable configurations.
Another waveguide approach includes the use of diode-pumped fiber lasers. In general, diode-pumped single-mode fiber lasers have demonstrated an ability to generate high beam quality laser radiation in which the diode pump radiation may be confined to a relatively large core as opposed to just the single-mode core that contains the active lasant material. This fiber approach has generated a recent resurgence of interest for applications requiring output powers of 10 W or more. Although, in principle, such fiber lasers can be q-switched, they are limited in their peak power generation by physical processes such as Raman scattering. The Raman scattering threshold, which for single-mode fibers that are several meters in length is on the order of 10 GW/cm.sup.2, may limit the pulse energy and pulse widths that can be generated using the fiber laser approach. If this Raman threshold is exceeded, the laser output may be dominated by the Raman spectrum resulting in a broad super-continuum. This may be unacceptable for the majority of applications that require either single frequency or narrow band output pulses.
The operation of the first glass or crystal planar (non-fiber geometry) waveguide laser is open to interpretation depending specifically on how the thickness of a thin waveguide is defined. The early demonstration of a laser using a 50 .mu.m thick LiNdP.sub.4 O.sub.12 gain element, pumped by an Ar-ion laser, may be considered to be the first open literature report of the successful operation of a system that took advantage of and recognized the potential for planar crystalline structures. (K. Kubodera, J. Nakano, K. Otsuka and S. Miyazawa, A slab waveguide laser formed of glass-clad LiNdP4O12, J. Appl. Phys. 49, pp. 65-68 (1978)). However, this initial demonstration employed a waveguide that was approximately an order of magnitude thicker than that which is generally required to support only single transverse mode operation in the waveguiding dimension. Furthermore, these waveguides were deposited by RF sputtering onto glass substrates and suffered from scattering losses. Since this initial demonstration, there have been many others using various laser systems to serve as pump excitation sources for various ion-host combinations. The most recent developments in the field of waveguide lasers have been associated with using semiconductor laser diodes as pump sources. Using such semiconductor pump sources, known pumping schemes for slab-type waveguide lasers have involved the use of coupling optics such as a spherical lens to focus a diode pump beam into the lasing core of the waveguide. The use of intermediate coupling optics between the semiconductor laser pump array and the waveguide core is warranted in many of the developed systems by the highly divergent nature of the laser diode light. Because of the high divergence of the radiation emitted by a diode bar, there is a need to collect and collimate the light so that it can be delivered to the laser waveguide core with some efficiency. Typically, reported waveguide lasers use simple two layer, or three layer structures, in which the lasing core is either located on one side, or positioned between, a cladding material that has slightly lower index of refraction than the core. This step in refractive index is what permits the confinement of both the pump radiation and the developed laser radiation. One disadvantage of waveguide lasers with laser diode pump excitation sources is that the requirement of generating an output beam with high beam quality runs counter to the requirement of achieving simple and efficient pump laser radiation confinement. This conflict basically arises because in order to achieve efficient and simple pump confinement, it is often desirable to have a confining structure with a high numerical aperture, or in other words, a confining structure defined by an interface with a large step in the value of refractive index from one side to the other. The larger the numerical aperture of the waveguide, the larger the spread in angle of pump radiation that will be confined by the waveguide. However, a structure with a large numerical aperture will support a larger number of transverse lasing modes beyond the desired diffraction limited or fundamental mode than will a structure with a small numerical aperture. Because the spatial quality of a laser beam can be related to the number of transverse modes it contains beyond the fundamental mode, it is generally desirable to limit the numerical aperture that serves to confine the developed laser radiation. Because the same confinement structure is generally used for both the pump and the developed laser radiation, it is often necessary to compromise waveguide laser designs in current configurations given that both output laser beam quality and pump radiation confinement cannot be independently optimized.